One thing though: aria.uci's user.tar.gz contain a symlink from /opt/aria2/bin/aria2 to /usr/bin/aria2.

This isn't a good idea because that symlink won't work unless mkwriteable has been run previously (ie another .dsl has been installed) and part of the idea of uci extensions is to avoid running mkwriteable as far as is possible.

Perhaps the maker might want to symlink that binary into /opt/bin? This is a better way to put it in PATH.